Etching composition and process



and its use. a novel etching composition containing dissolved metals United States Patent 3,253,968 E'ICHING COMPOSITION AND PROCESS Clayton C. Shepherd, .Ir., Bellllower, and John Derbyshire, Jr., Downey, Calif., assignors to North American Aviation, Inc.

No Drawing. Filed Oct. 3, 1961, Ser. No. 142,498 8 Claims. (Cl. 156-19) This invention relates to a novel etching composition More particularly, this invention relates to therein.

Etching compositions and methods have been developed for the chemical etching or processing of structural metal parts. This process is particularly suited for the manufacture of structural parts possessing high strengthto-weight ratio. Such structural parts are obtained by etching out areas undergoing the least mechanical stress to provide a thinner section at such a location. One of the requirements of etching by chemical means is the non-impairment of the mechanical properties of a struc: tural part for service conditions of fatigue and high stress. The use of etchant, must, therefore, not produce selective etching, channeling, rough surfaces, pitting, or intergranular attack. This invention developed as a result of a search for an improved etching composition and chemical milling process.

An' object of this invention is to provide a novel etching composition. Another object is to provide an improved method of processing structural parts. It is also an object of this invention to provide a method for etching metals without impairing the integrity of the part for service conditions under high stress and strain. It is also an object to provide an etchant composition and process which minimizes or eliminates selective etching, channeling, rough surfaces, pitting and intergranular attack. Still other objects of this invention will become apparent from the discussion which follows.

The above objects are accomplished by providing an etchant composition consisting essentially of an aqueous solution containing from about 0.5 to about 8 gram atoms of acidic hydrogen ion per liter, from about 1.6 to about 8.3 gram atoms dissolutioned metal ion per liter, said hydrogen and said dissolutioned metal ion being present in the form of 65-100 atom percent of soluble nitrates and chlorines, and 0-35 atom percent soluble phosphates, and the atom ratio of nitrogen-tochlorine in said nitrates and chlorides is from about 0.05:1 to about 4.5 :1, said dissolutioned metal 'ion containing chromium in an amount such that the ratio of total dissolutioned metaLto-chromium is from about 28:1 to about 2:1, said ratios being expressed in terms of numbers of gram atom equivalents of the respective components.

The above etchant composition is used for Chem-Mill processing workpieces of Group VIII metals and alloys thereof. The constituents of such Group VIII metals and alloys include iron, cobalt, nickel, as well as alloying elements such as chromium, molybdenum, titanium, manganese, carbon phosphorus, sulphur, zirconium, selenium, silicon, columbium, copper, aluminum, as well as small amounts of other metals. Non-limiting examples of steels that are chemically milled by the compositions of this invention include austenetic stainless steels of the AISI No. 300 type series, such as AISI type No. 301, 302, 303, 304, 308, 309, 316, 321, 347, etc., 4130, 17-7 PH steel, -7 Mo, Reni 41, Inconel, Inconel X, etc. The composition of the various steels will be found in the Metals Handbook, published by the American Society for Metals, Cleveland, Ohio. The l7-7 PH steel, for example, contains about 73 weight percent "ice iron, 17 weight percent chromium, 7 weight percent nickel, together with small amounts of other impurities. The 4130 steel contains about 97 weight percent iron, 1 weight percent chromium, 0.5 weight percent manganese, 0.3 weight percent carbon and small amounts'of other impurities. The Inoonel X contains about 7 wt. percent iron, wt. percent nickel, 15 wt. percent chromium, plus small amounts of other elements.

When the Group VIII elements and alloys thereof are milled by the etchant composition of this invention, a number of chemical milling problems are eliminated. Some of the eifects or undesirable conditions that are overcome include passivation, sandy surfaces, and channeling. By channeling is meant the etching away of a greater amount of metal from the surface exposed to the etching solution along the edge of the masking material which is employed on that portion of the surface which is being protected from contact with the etching solution. Acceptable products are obtained when the workpieces are milled by the etchant having the composition outlined above. When the components are varied outside a specified range, a number of undesirable effects are noted. When the concentration of the acidic hydrogen ion falls below 0.5 gram atoms per liter, passivation usually occurs unless the gram atoms of nitrogen are proportionately reduced. However, reaction rates at low gram atoms of hydrogen and nitrogen are, generally too low for production type applications unless the gram atoms of dissolutioned metal, as metal chlorides, are increased above 3.0. On the other hand, when the acidic hydrogen ion concentration exceeds about 8 gram atoms per liter, undesirable etching characteristics occur such as pitting, intergranular corrosion and uncontrollable reaction rates. High evaporation rates also occur at high concentrations of acidic hydrogen ion causing a too rapid depletion of the etchant solution for controlled etching of structural parts. When the amount of the dissolutioned metal, as metal chlorides in the etching solution, falls below 1.6 gram atoms liter; the activity of the etchant decreases. The metallic ions contribute a strong oxidizing effect which decreases with a decrease in the concentration of these ions. This can cause semipassivation, rough etched surface finishes, and low reaction rates unless the grams atoms of hydrogen and nitrogen are proportionately increased. When the amount exceeds 8.3 gram atoms per liter the solution becomes thick, viscous, high in specific gravity and sensitive to formation of precipitates. The high viscosity, high specific gravity solution is ditficult to work in a normal pumping system.

Sensitivity and rapid formation of precipitates can cause stoppage in piping and heat exchangers. Insoluble precipitates on parts being etched cause rough surfaces.

The hydrogen and dissolved metal in solution is prer dominantly present in the form of soluble nitrates and chlorides, particularly as chlorides. In some instances, soluble acid phosphates are also present. In the latter case, it is found that at least about 65 atom percent of the nitrogen, chlorine, and phosphorus in solution can be in the form of nitrates and chlorides. When this value falls below the 65 atom percent of nitrogen and chlorine, insoluble phosphates may occur which could cause uneven'etched surfaces by settling on the workpiece. Excessive phosphates, even as phosphoric acid, have caused dishing in the etched areas on the workpiece. The phosphoric acid may be eliminated from the etchant composition if high gram atoms of dissolutioned metal are desired.

The atom ratio of nitrogen-to-chlorine in the nitrates and chlorides solution is from about 0.1:1 to about 4.521. When the ratio falls below the 0.1:1 value the reaction rates become exceedingly slow except at high concentrations of acidic hydrogen ion. However, the lower nitrogen concentrations could deplete rapidly allowing passivation to occur at low dissolutioned gram atoms of metal ion during the process cycle. On the other hand, when a ratio becomes higher than a 4.5 :1 value, a strong oxidizing effect would exist but there would be insutficient chloride to complex the desired concentration of gram atoms of dissolved metal (3 gram atoms chloride per gram atom metal). This would also cause, in most cases, passivation, particularly with the high nickel-chromium, heat resistant alloys.

It has found that the dissolutioned metal must include, in part, chromium in order to minimize or eliminate certain undesired effects such as rough etched surfaces and semi-passivation of the nickel base alloys. Chromium ion contributes a strong catalytic effect, especially at low metallic ion concentrations. At high gram atoms of dissolutioned metal, chromium ion is not essentially re quired for activity but for desired etch characteristics such as surface finish, etc. Etching compositions containing a metal-to-chromium ratio of from about 28:1 to about 2:1 give good results. When the ratio exceeds the higher value of 28:1, the nickel base alloys, such as Inconel X, become passive. The passive condition is minimized at high total metal concentrations or, in some cases, by the additions of fluorides. The high total metal is difiicult to maintain and, on many alloys, fluoride causes a detrimental intergranular attack. When, on the other hand, the ratio falls below the value of 2: 1, the relative activity of the etchant increases and selective etching occurs in the areas adjacent to the masked edge. This channeling is most severe on the high nickel alloys and at the lower concentration of total dissolutioned metal.

Especially good results are obtained when the acidic hydrogen ion concentration is from about 2 to about 6 gram atoms per liter and the ratio of hydrogen ion-tometal solution is from about 0.3121 to about 1.25:1, the atom ratio of nitrogen-to-chlorine solution is from about 0.22:1 to about 1.611, and the ratio of total dissolutioned metal-to-chromium is from about 2821 to about 2:1. Etching solutionshaving compositions satisfying the above range requirements, therefore, constitute a preferred embodiment of this invention.

When the hydrogen ion concentration and dissolutioned metal concentration are within the above stated ranges, it is found that good products are obtained when the hydrogen ion-to-dissolved metal atom ratio has the value given. However, at the higher gram atoms of dissolutioned metal, this ratio becomes less critical such that satisfactory results are obtained even at ratios as great as 1:8.

The processing of workpieces of the Group VIII metals' and alloys thereof comprises exposing such a workpiece to the etching action of a solution initially having a composition falling within the limits described hereinabove. The composition of the etchant is maintained within these limits during the milling of the workpiece by routine chemical analysis and suitable bath additions.

The phosphorus when required or used, is present in the solution in the form of soluble metal hydrogen phosphates. Examples of such phosphates, are phosphoric acid, ferric phosphate and its hydrates, the hydrogen phosphates of iron, such as Fe (HPO and Fe(H PO chromic phosphate and its hydrates, chromic hydrogen phosphates, nickel phosphate, nickel hydrogen phosphates, etc. The dissolved metal is present in the form of metal solids, or as the metal phosphates named above or the soluble nitrate and chloride salts of the metals. Hence, the dissolved metal will be present in solution in a form of metal ions. The identity of the various metal ions in solution during and after processing of a metal workpiece will depend to a great extent on the composition of metal workpieces subjected to the action of the chemical milling bath. Usually the metallic ion constituents are mainly metal chlorides such as ferric chloride.

The nitrate and chloride ions are present in the form of soluble metal nitrates and chlorides, hydrochloric acid and/ or nitric acid. Small amounts of other components, including organic and inorganic acids such as sulphuric acid, nitrous acid, acidic and other organic acids, may also be present either as impurities or in definite amounts such as dodecyl benzene sulfonic acid, added to modify the chemical milling solution.

Various amounts of the mineral acids and water are lost in the course of carrying out the process due to evaporation and reaction mechanisms; dependent on etchant temperatures and rates of reaction. To compensate for this, additions of the various components re-,' quired in the chemical milling solution are made at specitic intervals in order to replace those lost and to main-' tain the relative proportions of the various components Within the ranges set out hereinabove.

The etching composition of this invention is compounded by adding the various components, including water, to a suitable vessel which is not affected by the composition, such as, for example, a container lined with rigid poly vinyl chloride; this solution is mixed until substantially homogeneous. A workpiece, having those surfaces which are not to be subjected to the action of the chemical milling solution suitably protected with a masking composition, is lowered into the etching solution and maintained therein for a period of time sufficient to provide for a predetermined depth of etch. During this time, the composition of the etching bath is maintained within the limits specified hereinabove by suitable addition of the various components. The amounts of the various components in the etching solution is determined by chemical analysis of a composite sample withdrawn from the bath at specified intervals.

The temperature of the chemical milling bath (etchant) is thermostatically maintained at a pre-determined value, sufficient to provide for a specifically desired etch rate on the particular Group VIII metal or alloy thereof that is being subjected to the Chem-Millprocess-ing. The temperature can range from about to about 160 F. providing the temperature variation throughout the etching does not vary more than a few degrees F. Satisfactory etching rates of from about 0.020 to about 0.090 inch per hour is obtained at a temperature of about F. The carrying out of processing'at this temperature constitutes a preferred embodiment of this invention. However, the required temperature and limits thereof, are based upon (1) etch rates required for production processing, (2) volatization of the mineral acids compounded in the etchant, (3) the temperature required for effective dissipation of the heat evolved from the reaction when metal is dissolutioned and (4) the specific limits of processing equipment such as tank liners.

The following non-limiting examples illustrate the composition and process of this invention.

Example I To a milling vessel made of mild stainless steel and lined with an inert liner such as rigid poly vinyl chloride, nitric acid, phosphoric acid, ferric chloride, ferric phosphate, and chromic chloride and water, when required, were added in amounts such as to provide 2.5 gram atoms of acidic hydrogen ions per liter, 3.5 gram atoms of nitrogen in the form of nitrate ions per liter, 7.4 gram atoms of chlorine in the form of chloride ions per liter, 4.5 gram atoms of phosphorus in the form of soluble phosphates per lite-r, 2.1 gram atoms of total metal in the form of soluble metal salts, and 0.3 gram atoms of chromium. The mixture was agitated until a substantially homogenelous composition was obtained. The vessel Was equipped with heating means and the temperature of the solution was brought up to substantially 150 F. A workpiece of 17-7 PH steel, having a portion of its surface masked with an acid resistant coating (such as butyl rubber compounds), was then suspended in the milling solution and the solution allowed to act thereon until the metal of the exposed surface had been etched away to a depth of substantially 0.125 inch. The time required for this was about two hours, providing an etching rate of substantially 0.060 inch per hour. Prior to, during, and at intervals of approximately four hours milling solution was with drawn and analyzed for (1) acidic hydrogen, soluble nitrates, chlorides and phosphates and (2) dissolutioned metal ion per liter. Hydrochloric and nitric acid, and water when required, were added in amounts sufiicient to maintain the hydrogen ion concentration from about 0.5 to about 8 gram atoms per liter, the amount of dissolved metal at from about 1.6 to about 8.3 gram atoms per liter, and the nitrogen-to-chlorine ratio of from about 0.1:1 to about 4.5:1. Specific etchant compositions, dependent upon the nature of the workpiece are given in Table I. When the desired depth of etch had been obtained, the workpiece was removed and rinsed in water. The surface of the etched portion of the workpiece was smooth and milled to an even depth.

Table I discloses etchant solutions having compositions within the required ranges of concentration specified hereinabove in order to obtain a good product.

and had a specific gravity of 1.42. Two kinds of hydrochloric acid were used, of which one was 37 percent acid by weight having a specific gravity of 1.18, and the other was percent acid by weight and having a specific gravity of 1.15.

Although the invention has been described and illustrated in detail it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

1. An etching composition consisting essentially of an aqueous solution the solute of which consists essentially of ionizable ingredients, the anion constituents of said ingredients being hydrogen ions and metal ions respectively, said metal ions being hereinafter referred to as dissolved metal, said dissolved metal comprising chromium and iron, the solution containing from about 0.5 to about 8 grams atoms of acidic hydrogen ion per liter, from about 1.6 to about 8.3 gram atoms dissolved metal per liter, said hydrogen and said dissolved metal being present in the form of TABLE I.-COMPONENTS IN SOLUTION IN GRAM-ATOMS PER LIIER Example Workpieces 1 11+ 2 N 03- Cl- HPO, Metal Total Or NO /Cl H /M M/Cr 2 Uncombined hydrogen.

The phosphorous is present in the form of phosphate ions in the examples of this Writing. The various runs shown in Table I illustrate the use of both high and low hydrogen ion concentrations, with various nitrate, chlo' ride and phosphate concentrations within the prescribed gram-atom ranges of 0.5-8.0 H, 0.5-6.0 N0 2.023.0 Cl, 1.6-8.3 metal ion of Fe, Mo, Ni, Co with the prescribed ratio of Cr being present, and 0-45 phosphate. For example, the metal other than chromium in Run IV consisted of 2.7 gram atoms of iron. In Run VIII, the metal other than the chromium consisted of 1.0 gram atom of molybdenum. In Run XI the metal was composed of 1.3 gram atoms of iron, 0.3 gram atom of cobalt, and 0.1 gram atom of chromium. In the other runs, the metal other than the chromium consisted of iron. The temperature of the chemical milling bath was maintained at substantially 150 F. during the milling procedure but temperature variations can be made. The depth to which the various workpieces were milled ranged from 0.020 inch to 0.100 inch. The surface of the milled portion of the workpiece in each case was smooth, etched to an even ,depth,-with an absence of any channeling. When the components fall outside of the specified ranges set out hereinabove, the milled surface of the workpiece subjected to a particular solution is unacceptable for the reasons given earlier. The uniformity of chemically milled surfaces is of extreme importance in the manufacture of structural metal parts as, for example, in the manufacture of airframe structural panels having maximum strengthto-weight ratios. In the latter instance, the structural panels are pre-formed to the desired curvature of shape and then the areas undergoing the least mechanical stress are etched out to thinner sections of specified thicknesses with a highly quality to the chemically milled surface.

The acids used in this process are commercially available. The phosphoric acid used was either 85 percent acid by weight having a specific gravity of 1.69, or 75 percent acid by weight having a specific gravity of 1.58. The nitric acid employed was 70 percent by weight acid 65l00 atom percent of soluble nitrates and chlorides and 0-35 atom percent of solution phosphates, and the atom ratio of nitrogen-to-chlorine in said nitrates and chlorides is from about 0.05 :1 to about 4.5 1, said chromium being present in an amount such that the ratio of total dissolved metal-to-chromium is from about 28:1 to about 2:1, said ratios being expressed in terms of numbers of gram atom equivalents of the respective constituents.-

2. An etching composition consisting essentially of an aqueous solution the solute of which consists essentially of ionizable ingredients, the anion constituents of said ingredients being hydrogen ions and metal ions respectively, said metal ions being hereinafter referred to as dissolved metal, said dissolved metal comprising chromium and iron, the solution containing from about 0.5 to about 8 gram atoms of acidic hydrogen ion per liter, from about 1.6' to about 8.3 gram atoms dissolved metal per liter, said hydrogen and said dissolved metal being present in the form of 65 to atom percent of soluble nitrates and chlorides and 0 to 35 atom percent of phosphates,and the atom ratio of nitrogen-to-chlorine in said nitrates and chlorides is from about 0.05:1 to about 4.5 :1, the ratio of hydrogen ion-to-metal in solution is from about 0.31:1 to about 1.25:1 said chromium being present in an amount such that the ratio of total dissolved metal-to-chrominum is fromabout 28:1 to about 2:1, said ratios being expressed in terms of numbers of gram atom equivalents of the respective constituents.

3. An etching composition consisting essentially of an aqueous solution the solute of which consists essentially of ionizable ingredients, the anion constituents of said ingredients being hydrogen ions and metal ions respectively, said metal ions being hereinafter referred to as dissolved metal, said dissolved metal comprising chromium and iron, the solution containing from about 2 to about 6 gram atoms of acidic hydrogen ion per liter, from about 1.6 to about 8.3 gram atoms dissolved metal per liter, said hydrogen and said dissolved metal being present in the form of 65 to 100 atom percent of soluble nitrates and chloride-s and O to 35 atom percent of soluble phosphates, and wherein the atom ratio of said nitrogen-to-chlorine is from about 0.22:1 to about 1.6:1, said chromium being present in an amount such that the ratio of total dissolved metal-to-chrornium is from about 28:1 to about 2.5 :1, said ratios being expressed in terms of numbers of gram atom equivalents of the respectively constituents.

4. An etching composition consisting essentially of an aqueous solution the solute of which consists essentially of ionizable ingredients, the anion constituents of said ingredients being hydrogen ions and metal ions respectively, said metal ions being hereinafter referred to as dissolved metal, said dissolved metal comprising chromium and iron, the solution containing from about 2 to about 6 atoms of acidic hydrogen ion per liter, from about 1.6 to about 8.3 gram atoms dissolved metal per liter, said hydrogen and said dissolved metal being present in the form of 65 to 100 atom percent of soluble nitrates and chlorides and to 35 atom percent of soluble phosphates and wherein the concentration of said nitrate ions is from about 2 to about 13 gram equivalents per liter and the concentration of said chloride ions is from about 3 to about 13 gram equivalents per liter, and the atom ratio of nitrogen-to-chlorine in said nitrates and chlorides is from about 0.22:1 to about 1.6:1, the ratio of hydrogen ion-to-metal in solution is from about 0.31:1 to about 1.25:1, said chromium being present in an amount such that the ratio of dissolved metal-t o-chromium in solution is from about 28:1 to about 2.5:1, said ratios being eX- pressed in terms of numbers of gram atom equivalents of the respective constituents.

5. A process for chemically milling workpieces of Group VIII metals and alloys thereof comprising contacting a workpiece with an etching bath initially having the composition of claim I and maintaining the concentration of said ingredients within the ranges specified in claim 1 7 throughout the contacting step.

6. A process for chemically milling workpieces of Group VIII metals and alloys thereof comprising contact ing a workpiece with an etching bath initially having the composition of claim 2 and maintaining the concentration of said ingredients within the ranges specified in claim 2 through the contacting step.

7. A process for chemically milling workpieces of Group VIII metals and alloys thereof comprising contacting a workpiece with an etching ba t-h initially having the composition of claim 3 and maintaining the concentration of said ingredients within the ranges specified in claim 3 throughout the contacting step.

8. A process for chemically milling workpieces of Group VIII metals and alloys thereof comprising contact-.

ing a workpiece with an etching bath initially having the composition of claim 4 and maintaining the concentration of said ingredients within the ranges specified in claim 4 throughout the contacting step.

References Cited by the Examiner UNITED STATES PATENTS 1,545,498 7/1925 Klinger et al. 252- XR 2,890,944 6/1959 Hays 15619 2,940,837 6/1960 Acker et al 156-18 3,108,919 10/1963 Bowman et al 156- 18 JULIUS GREENWALD, Primary Examiner.

J. T. FEDIGAN, Assistant Examiner. 

1. AN ETCHING COMPOSITION CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION THE SOLUTE OF WHICH CONSISTS ESSENTIALLY OF IONIZABLE INGREDIENTS, THE ANION CONSTITUENTS OF SAID INGREDIENTS BEING HYDROGEN IONS AND METAL IONS RESPECTIVELY, SAID METAL IONS BEING HEREINAFTER REFERRED TO AS DISSOLVED METAL, SAID DISSOLVED METAL COMPRISING CHROMIUM AND IRON, THE SOLUTION CONTAINING FROM ABOUT 0.5 TO ABOUT 8 GRAMS ATOMS OF ACIDIC HYDROGEN ION PER LITER, FROM ABOUT 1.6 TO ABOUT 8.3 GRAM ATOMS DISSOLVED METAL PER LITER, SAID HYDROGEN AND SAID DISSOLVED METAL BEING PRESENT IN THE FORM OF 65-100 ATOM PERCENT OF SOLUBLE NITRATES AND CHLORIDES AND 0-35 ATOM PERCENT OF SOLUTION PHOSPHATES, AND THE ATOM RATIO OF NITROGEN-TO-CHLORINE IN SAID NITRATES AND CHLORIDES IS FROM ABOUT 0.05:1 TO ABOUT 4.5:1, SAID CHROMIUM BEING PRESENT IN AN AMOUNT SUCH THAT THE RATIO OF TOTAL DISSOLVED METAL-TO-CHROMIUM IS FROM ABOUT 28:1 TO ABOUT 2:1, SAID RATIOS BEING EXPRESSED IN TERMS OF NUMBERS OF GRAM ATOM EQUIVALENTS OF THE RESPECTIVE CONSTITUENTS.
 5. A PROCESS FOR CHEMICALLY MILLING WORKPIECES OF GROUP VIII METALS AND ALLOYS THEREOF COMPRISING CONTACTING A WORKPIECE WITH AN ETCHING BATH INITIALLY HAVING THE COMPOSITION OF CLAIM 1 AND MAINTAINING THE CONCENTRATION OF SAID INGREDIENTS WITHIN THE RANGES SPECIFIED IN CLAIM 1 THROUGHOUT THE CONTACTING STEP. 